1. Field of the Invention
The present invention relates to liquid crystal display devices and more particularly to MIM type nonlinear pixel driving elements and a method of manufacturing lateral MIM elements with substantially uniform operating characteristics. The invention further relates to a pixel driver element structure that provides improved operating characteristics in liquid crystal display devices.
2. Description of Related Art
Metal-insulator-metal (MIM) diodes are a common type of nonlinear element used to provide or control the driving voltage for signal conductors corresponding to individual pixels in liquid crystal displays and similar optical devices. A top view of a typical metal-insulator-metal (MIM) element is illustrated in FIG. 4 where it is shown being formed on a transparent element substrate 27 as a pixel driver element. A cross section taken along line I--I' extending through the same MIM element is shown in FIG. 5. Both of these figures show the use of a lateral (side surface) MIM structure in which only the side surface of an input signal conductor or electrode 21 is utilized as an input portion of the active pixel driver or control element. The MIM element is shown having a single output conductor 25 connected to one pixel electrode or conductor 26.
In forming conventional lateral MIM elements, such as illustrated in FIG. 4, all surfaces except the side surface 22 of the first signal conductor 21, i.e., where the MIM diode conduction occurs, are covered with a layer of insulating material to provide an electrical barrier layer 23. The electrical resistance of the barrier layer 23 is made sufficiently large so that it does not allow any other portion of the conductor 21 to function as part of the MIM diode. A lower resistance, typically thinner, insulating layer 24 is disposed on the side surface of the conductor 21 where the active MIM element is to be formed.
As seen in FIG. 5, material forming a second, output signal, conductor 25 for the nonlinear element is disposed on top of the insulating layers 24 and 23, and connects to the corresponding pixel electrode 26. A MIM diode is formed in the region where the first conductor 21, the insulator 24, and the second conductor 25 overlap and becomes the pixel voltage driving element for the pixel electrode 26 for one pixel of an associated display device. An array of such MIM driver elements is generally formed across the surface of the transparent element substrate 27 to achieve the desired optical display size and control.
This type of MIM structure, using known manufacturing techniques, allows the surface area of the MIM driver element to be made extremely small. This has proven to be an effective technology for increasing the density of pixel drivers and electrodes, and the manufacturing precision for liquid crystal display devices using MIM elements.
Several materials found useful in manufacturing the conductors for MIM elements include tantalum, aluminum, gold, ITO, NiCr+Au, and ITO+Cr, while useful insulator materials include TaO.sub.x, SiO.sub.x, SiN.sub.x, SiO.sub.x N.sub.y, TaN.sub.x and ZnO.sub.x. The insulating layers are generally formed by thermal or anodic oxidation, or by sputtering. In addition to inorganic compounds, polyimides and other organic materials can also be used in forming insulating barrier layers.
The most common structure used in manufacturing MIM driver elements is one that employs tantalum (Ta) for the first conductor 21, an oxide of tantalum (TaO.sub.x) for the insulator 24, and chrome (Cr) for the second conductor, resulting in a Ta/TaO.sub.x /Cr element structure.
However, using the conventional lateral MIM structure and manufacturing technique results in deviations in the characteristics of many of the MIM driver elements within a given optical display device. Because the lateral MIM element is formed on the side surface of the first conductor, where overlapped by the insulator 24 and second conductor 25, the surface area of the MIM element is proportional to the film thickness of the first conductor 21 and the angle formed between the side surface 22 and the transparent substrate 27, i.e., the cross sectional shape or taper of the side surface. Therefore, the area of the MIM element is very sensitive to variations in the film thickness or the side angle of the first conductor. Unfortunately, with current production process technology, the film thickness is often inconsistently distributed across a substrate which results in deviations in the operating characteristics of MIM elements distributed across the surface. The inability to precisely control MIM element surface area during the production process leads to inadequate control of pixel display conditions throughout a liquid crystal or similar type display device, that is, in the final product.
What is needed is a pixel voltage driver assembly using an array of MIM elements which provides substantially uniform device characteristics across the array, and, thus, across the optical device. It would also be beneficial if the MIM elements can be manufactured by a process of minimum complexity and in association with a variety of pixel driver applications.